Pre-formed, non-polarizable electrodes are commonly used to manufacture different DEL HES', in particular, capacitors having a PbO2|H2SO4|C system. In a HES having a PbO2|H2SO4|C system, use is made of a positive electrode with lead dioxide (PbO2) active material, such as is shown, for example, in U.S. Pat. Nos. 6,195,252 and 6,842,331. Electrodes with a lead dioxide active material are used as positive electrodes for lead-acid batteries, and are currently manufactured using various methods. A mixture of metal lead (Pb) powders and different lead oxides (Pb3O4 and PbO) are frequently used for the manufacture of positive electrodes with PbO2 active material. A paste is manufactured by mixing powders and an aqueous solution of sulfuric acid. The resulting paste is puttied in a positive electrode grid and then dried. After the electrode is dried it is formed. As a general rule, the amount of lead sulfate (PbSO4) in the dry non-formed active material does not exceed 5%. The formation process makes it possible to oxidize all the active material to a lead dioxide.
Positive electrodes are typically formed by one of two main methods. According to a first method, positive electrode plates are formed in combination with negative electrodes before the associated lead acid battery or HES having a PbO2|H2SO4|C system is formed. In order to manufacture lead-acid batteries, both positive and negative pre-formed electrodes are used. In the manufacture of a HES, pre-formed positive electrodes are used.
According to a second method, positive and negative electrodes are formed after assembly of a battery. The second method is primarily used for the manufacture of advanced lead-acid batteries, and is easier, ecologically safer, and less costly than the first method.
When a formation method for a positive electrode is available, it is possible to use positive electrodes pre-formed and made of lead oxide. However, this method is not optimal, and does not allow for the manufacturing of capacitors with high and stable specific energy, capacity parameters, or improved cost. When a HES with pre-formed positive electrodes is assembled, the positive electrodes are fully charged, while the negative electrodes are fully discharged. To provide for serviceability of a capacitor, both the positive and negative electrodes need to be in a fully charged state. Therefore, when a capacitor is manufactured using formed positive electrodes, an additional process is performed after assembly to balance the Coulombic capacities of the capacitors' positive and negative electrodes.
The technology and process of balancing the Coulombic capacities of capacitor electrodes' depends upon the design and overall dimensions of the electrodes. In addition, known techniques for balancing Coulombic capacities require labor-intensive, costly and long procedures. Because high rates of oxygen recombination are typical of all HES' having an aqueous electrolyte, the process of balancing sealed capacitors becomes more complicated. In particular, a HES having a PbO2|H2SO4|C system where the porous carbon DEL electrode is negative, is quite complicated. High oxygen recombination in the capacitor's negative electrode results in high efficiency of the oxygen cycle and impedes the evolution of oxygen from the capacitor during its charge, which in turn appreciably decreases the efficiency of the balancing process.
In one known method of forming and charging a DEL capacitor negative polarizable carbon electrode, a HES is manufactured using a pre-formed positive electrode with a PbO2/PbSO4 active material and a negative electrode based on activated carbon materials. An aqueous solution of sulfuric acid is used as an electrolyte. The capacitor, prior to its sealing, is placed in a sealed chamber through which an inert gas flow (nitrogen or argon) is circulated. Thereafter, the capacitor is charged. The process of charging the capacitor is complete when the potential of the negative electrode reaches a pre-set value. After the potential reaches this pre-set value, the capacitor is sealed. During the charging phase, the positive electrode generates oxygen, and the negative electrode produces hydrogen. To provide effective removal of hydrogen and oxygen from the capacitor, separators and/or electrodes having extended channels are used.
This method of forming and charging a DEL capacitor negative polarizable carbon electrode has many drawbacks that prevent the manufacture of capacitors with high specific energy parameters and low cost. This method of capacitor manufacturing does not take into account the current state of the art with respect to the manufacture of PbO2/PbSO4 positive electrodes. This omission makes it impossible to manufacture commercial capacitors having wide applications. The aforementioned capacitor design allows for only one plate of the positive electrode to be used; making it impossible to manufacture a capacitor with high capacitance and discharge energy. To manufacture capacitor systems having high discharge energies by this known method, it is required to connect many capacitor cells, which brings about an abrupt deterioration of the specific parameters, an increase in cost and a decrease in the reliability of capacitor system operation.
When capacitors having a PbO2|H2SO4|C system are manufactured as described above, the pre-formed positive electrode is fully charged. However, the negative electrodes are in a discharged state. To provide a functional capacitor after assembly, the Coulombic capacities of the positive and negative electrodes need to be balanced. To make balancing the Coulombic capacities possible, the negative electrode is overcharged. Because the Coulombic capacity of the positive electrode is considerably higher than that of the negative electrode, the process must be repeated several times. This need to repeat the balancing step increases the time and cost of the capacitor manufacturing process.
Since the positive electrodes are in a charged state during the initial stage of the Coulombic capacity balancing step, oxygen is evolved in the positive electrode. The rate of the oxygen evolution increases along with a growth of the positive electrode's state of charge. The oxygen diffuses to the negative electrode and discharges it, which prevents the balancing process. As a result of this discharge, the negative electrode is not fully charged, and the specific energy and capacity parameters of the capacitor do not reach their maximum values. To effectively balance the Coulombic capacities of the positive and negative electrode it is necessary to charge the capacitor under conditions that ensure a full charge of the negative electrode. For a maximum charge of the negative electrodes of a HES, it is necessary to remove the oxygen from the capacitor. The efficiency of the Coulombic capacity balancing process depends upon the efficiency of the oxygen removal. The oxygen removal process is particularly important for capacitors having large electrodes.
According to the previously described known method, the oxygen is removed during the Coulombic capacity balancing process by a flow of inert gas. This method of oxygen removal has several drawbacks. First, an inert gas flow may effectively remove the oxygen only from the periphery of the electrodes. Since the oxygen creates excessive pressure inside the capacitor case, the penetration of the inert gas flow into the central portion of the electrodes is impeded. Even high intensity inert gas flow will not effectively remove oxygen from the central portion of the capacitors. A non-uniform removal of oxygen from the capacitor will result in a non-uniform charge of the negative electrodes.
Therefore, this method of oxygen removal does not provide efficient balancing of electrode capacities across a broad range of capacitor designs and sizes. It may only be effective for capacitors having one positive plate and two negative plates of small dimensions. Additionally, the use of an inert gas flow makes it impossible to perform an effective and uniform removal of oxygen during a Coulombic capacity balancing process. Thus, the use of this known method will result in a strong and non-uniform heating of the capacitor, which will have a negative affect on the parameters of the non-polarized electrodes thereof.
Second, the inert gas may partially fill the pores of DEL capacitor negative electrodes by displacing the electrolyte. As a result, the polarization resistance of the polarizable electrodes may increase while charging the electrodes. Additionally, the potential of DEL capacitor negative electrodes may appreciably shift toward lower values at low levels of charge. Consequently, the manufacturing method described above does not allow for: 1) an accurate and correct determination of the charge of the negative electrode; 2) a full balancing of an electrode's capacities; or 3) a high and maximum specific parameters of a HES.
In order to manufacture capacitors with high specific energy and capacity parameters, activated carbon materials with large, developed areas are frequently used. A large developed surface area may be covered by pores having a diameter of about 0.5-3 nm. Another portion of the surface is covered by pores of small dimension making it difficult to fill with electrolyte. Therefore, the value of the capacitance of activated carbon DEL capacitor polarizable electrodes depends considerably on the rate of the filling of the electrodes' pores by the electrolyte. An effective filling of the polarizable electrodes' pores by the electrolyte is important to provide for maximum energy and capacity parameters and an appropriate balance of the Coulombic capacities. Depending on the parameters of the porous structure and manufacturing process of the DEL capacitor electrodes, the filling of small size pores by the electrolyte may take several days under normal conditions. To speed up this process, different methods of carbon electrode wetting by the electrolyte are employed.
The values of the polarization resistance and potential of DEL capacitor electrodes are related to the filling of the pores by the electrolyte. A partial filling of the pores results in a DEL capacitor electrode having elevated values of the polarization resistance and the potential's polarization. Therefore, during the charge of a HES there occurs a rather strong polarization of the potentials toward the negative area of only the near surface layers of the DEL capacitor negative electrodes, while the deeper layers of the electrodes are charged to a lesser extent. (See, e.g., S. A. Kazaryan et al., J. Electrochem. Soc., 153 (9), A1655-A1671, 2006).
In this case, immediately after the charge current of the capacitors is turned off, the potential of the negative electrodes shift unevenly toward positive values. Due to this effect, there occurs decomposition of the electrolyte and evolution of hydrogen in the capacitor's negative electrode in the area of the potentials of the electrolyte's thermodynamic stability. (See, e.g., B. Pillay and J. Newman, J. Electrochem. Soc., Vol. 143, No 6, 1996). A similar effect takes place when oxygen is removed from capacitors by an inert gas flow. A non-uniform effect of the inert gas on the DEL capacitor electrodes results in uneven distribution of the charge current density and the potential of the electrodes along the surface area and thickness. In the portions of the DEL capacitor electrodes where there is a strong effect of the inert gas, the potential decreases and hydrogen is evolved. The hydrogen displaces the electrolyte from the electrodes' pores. This, in turn, brings about further growth of the polarization resistance of the polarizable electrodes, which has a negative effect on the parameters of the capacitors and the process of balancing the Coulombic capacities.